The production of methanol from carbon monoxide and hydrogen in contact with catalysts is a highly exothermic reaction, which involves a volumetric contraction.
The commercial synthesis of methanol has been carried out for a long time in high-pressure reactors under pressures of 100-300 kilograms per square centimeter and at temperatures of about 320.degree.-380.degree.C. in contact with catalysts which contain oxides of zinc and chromium. To enable a dissipation of the heat of reaction, the catalyst in the high-pressure reactor is divided into layers and cold fresh synthesis gas is added to the reaction mixture between these layers. It is also known to provide heat exchange elements, which are disposed in or between the catalyst layers through which flows a cooling fluid. The utilization of the surplus heat of reaction by means of such heat exchange elements involves a high structural expenditure and also involves considerable difficulties owing to the high temperature level.
Copper-containing catalysts having a higher activity have recently been developed. Methanol can be synthesized in contact with these catalysts under pressures less than 100 kilograms per square centimeter (absolute pressure) and at temperatures of 230.degree.-270.degree.C. Medium-pressure equipment may be used for reaction under such conditions.
This means, for example, that piston compressors may be replaced by rotating compressors and considerable savings of material are enabled in the overall plant.
In a special embodiment of that process, the catalyst is disposed in the tubes of a tubular reactor, which is indirectly cooled by water boiling under super-atmospheric pressure. In this way the heat of reaction to be dissipated is utilized as high-grade energy in the form of high-pressure steam.
Synthesis gases for the production of methanol consist of H.sub.2 and CO and may contain CO.sub.2. They must be free of sulfur and should contain as little inert gas as possible. Inert gases in the reaction to produce methanol are nitrogen, argon and methane. Suitable synthesis gases may be produced in a tube furnace by a cracking of gaseous hydrocarbons or of liquid evaporable hydrocarbons having a final boiling point of about 200.degree.C. by a treatment with water vapor in contact with indirectly heated catalysts which contain nickel and under pressures of 5-30 kilograms per square centimeter and at temperatures of 750.degree.-900.degree.C. Because the feedstocks, preferably natural gas or light gasoline, are in most cases available in a sulfur-free condition, the cracked gas obtained as a primary product can be compressed to the synthesis pressure when the water vapor has been removed from the gas by condensation whereas a special purification is not required. The compressed cracked gas can be introduced into the reactor.
The synthesis of methanol is a large-scale chemical process in which low-cost feedstocks are preferred. For this reason a considerable part of the synthesis gases containing CO and H.sub.2 is produced by partial oxidation, suitably in the presence of steam, from heavy hydrocarbon oils, such as heavy fuel oils or residual oils obtained by the distillation or cracking of petroleum. Pure oxygen is required in these processes if the synthesis gas has to be free of inert gases. Besides, the cracked gas obtained as a primary product contains sulfur. It has been found that the synthesis of methanol carried out under a pressure of 30-60 kilograms per square centimeter above atmospheric pressure and at a temperature of 240.degree.-270.degree.C. can be combined in a highly desirable manner with the production of synthesis gas by a partial oxidation treatment of heavy hydrocarbon oils with pure oxygen in the presence of water vapor, and with the required purification of the gas, if the synthesis itself is carried out in a catalyst which is indirectly cooled by water boiling under superatmospheric pressure and the raw synthesis gas is produced under a pressure which is at least 5 and preferably 10-15 kilograms per square centimeter higher than the pressure of the methanol synthesis. In this process, the need for a compression of the synthesis gas before the synthesis reactor is eliminated and the high-pressure steam produced in said reactor is available to supply the compression energy which is consumed in the process. By expanding this steam to atmospheric pressure, about 270 kWh per metric ton of methanol product can be generated. This is more than one-half of the compression energy required in the overall process and about 70% of the compression energy required for the production of the compressed pure oxygen by a low-temperature separation of air.
Additional high-pressure steam is available from the waste heat boiler associated with the gas producer and may be used for the gasifying reaction and for the conversion of the carbon monoxide in a partial stream of the raw gas to carbon dioxide and hydrogen. The remaining surplus of said high-pressure steam may be used for the production of power. Any residual amount of steam which is required may be produced in a suppplementary boiler, which is fired with the by-products of the methanol synthesis, such as dimethyl ether, the residual synthesis gas (purge gas), the first runnings obtained by the distillation of methanol, and the soot-oil obtained by the purification of the raw synthesis gas. The supplementary boiler serves also to superheat the steam required for a production of power.